Spinodal decomposition and discontinuous precipitation in a low‑solvus Au‑Pt‑Pd alloy: an in situ X‑ray diffraction study

Spinodal decomposition and discontinuous precipitation are critical phase transformations that influence the mechanical properties of Au‑Pt‑based alloys. In this study, the microstructural evolution of a developmental Au-Pt-Pd alloy during ageing has been characterised in situ using high-resolution synchrotron X-ray diffraction. This alloy was designed to have a low solvus temperature, while retaining significant age‑hardenability. For the ageing temperatures studied (560 °C, 653 °C, and 741 °C), sideband peaks developed at the leading and trailing edges of the fundamental Bragg reflections, which are characteristic of a modulated structure formed by spinodal decomposition. With further ageing, the sidebands and parent Bragg reflections were progressively replaced by peaks from two face-centred cubic phases with different lattice parameters. Examination of the aged microstructure by electron microscopy revealed that these phases formed co‑operatively at grain boundaries by discontinuous precipitation and consumed the pre-existing modulated structure. The temporal evolution of the diffraction data indicated that the modulated structure underwent power‑law coarsening while the kinetics of discontinuous precipitation were well described by the Avrami model for nucleation and growth. Additionally, anisotropic and asymmetric broadening of diffraction peaks observed in the as‑quenched condition was attributed to coherency strains arising from elemental clustering, as detected by atom probe tomography. These findings demonstrate the utility of high-resolution synchrotron X-ray diffraction for gaining previously inaccessible insights into the precipitation phenomena that relate composition, microstructure, and crystallography in phase-separating systems.